Angled wafer rotating ion implantation

ABSTRACT

Ion implantation by mounting a semiconductor wafer on a rotating plate that is tilted at an angle relative to an ion implantation flux. The tilt angle and the ion implantation energy are adjusted to produce a desired implantation profile. Ion implantation of mesa structures, either through the semiconductor wafer&#39;s surface or through the mesa structure&#39;s wall is possible. Angled ion implantation can reduce or eliminate ion damage to the lattice structure along an aperture region. This enables beneficial ion implantation profiles in vertical cavity semiconductor lasers. Mask materials, beneficially that can be lithographically formed, can selectively protect the wafer during implantation. Multiple ion implantations can be used to form novel structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

UNITED STATES GOVERNMENT RIGHTS

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ion implantation of semiconductor devices.More particularly, this invention relates to ion implantation techniquesthat are suitable for ion implanting vertical cavity surface emittinglasers and that can result in novel implantation structures.

2. Discussion of the Related Art

Vertical cavity surface emitting lasers (VCSELs) represent a relativelynew class of conductor lasers. While there are many variations ofVCSELs, one common characteristic is that they emit light perpendicularto a wafer's surface. Advantageously, VCSELs can be formed from a widerange of material systems to produce specific characteristics. Inparticular, the various material systems can be tailored to emitdifferent wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and soon.

VCSELs include semiconductor active regions, distributed Bragg reflector(DBR) mirrors, current confinement structures, substrates, and contacts.Because of their complicated structure, and because of their materialrequirements, VCSELs are usually grown using metal-organic chemicalvapor deposition (MOCVD) or molecular beam epitaxy (MBE).

FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped galliumarsenide (GaAs) substrate 12 has an n-type electrical contact 14. Ann-doped lower mirror stack 16 (a DBR) is on the substrate 12, and ann-type graded-index lower spacer 18 is disposed over the lower mirrorstack 16. An active region 20, usually having a number of quantum wells,is formed over the lower spacer 18. A p-type graded-index top spacer 22(another confinement layer) is disposed over the active region 20, and ap-type top mirror stack 24 (another DBR) is disposed over the top spacer22. Over the top mirror stack 24 is a p-type conduction layer 9, ap-type GaAs cap layer 8, and a p-type electrical contact 26.Alternately, the top mirror and graded-index region can consist of atunnel junction structure. This comprises a short p-doped region nearestthe active region junction. Beyond the p-doped region is a tunneljunction followed by an n-type DBR.

Still referring to FIG. 1, the lower spacer 18 and the top spacer 22separate the lower mirror stack 16 from the top mirror stack 24 suchthat an optical cavity is formed. As the optical cavity is resonate atspecific wavelengths, the mirror separation is controlled so as toresonant at a predetermined wavelength (or at a multiple thereof). Atleast part of the top mirror stack 24 includes an insulating region 40formed by implanting ions (protons or certain other elements such asdeuterium, helium, iron, etc.) that provides current confinement.Protons can be implanted, for example, in accordance with the teachingsof U.S. Pat. No. 5,115,442, which is incorporated by reference.Alternatively, the insulating region 40 can be formed using an oxidelayer, for example, in accordance with the teachings of U.S. Pat. No.5,903,588, which is incorporated by reference. However, the principlesof the present invention relate to insulating via an ion implantingprocess. In either event, the insulating region 40 defines a conductivecircular central opening 42 that forms an electrically conductive paththrough the insulating region 40.

In operation, an external bias causes an electrical current 21 to flowfrom the p-type (or n-type in the case of a tunnel junction device)electrical contact 26 toward the n-type electrical contact 14. Theinsulating region 40 and the conductive central opening 42 confine thecurrent 21 such that it flows through the conductive central opening 42to the active region 20. Some of the electrons in the current 21 areconverted into photons in the active region 20. Those photons bounceback and forth (resonate) between the lower mirror stack 16 and the topmirror stack 24. The lower mirror stack 16 and the top mirror stack 24are very high reflectivity mirrors, with the lower mirror being at leastslightly higher reflectivity than the top mirror. Due to thisdifference, some photons emerge as coherent light 23, i.e., laser. Stillreferring to FIG. 1, the light 23 passes through the p-type conductionlayer 9, through the p-type GaAs cap layer 8, through an aperture 30 inthe p-type electrical contact 26, and out of the surface of the verticalcavity surface emitting laser 10. In the tunnel junction device format,the light follows the same path but through the tunnel junction layerand the n-type top mirror.

It should be understood that FIG. 1 illustrates a common VCSELstructure, and that numerous variations are possible. For example, thedopings can be changed (say, by providing a p-type substrate 12),different material systems can be used, operational details can be tunedfor maximum performance, and additional structures, such as tunneljunctions (briefly described above), can be added.

In addition, it should be noted that multiple VCSELs can be formed onthe same substrate, thus producing a VCSEL array. Ion implantingprovides a method of electrically isolating individual VCSEL elements.To do so, ions with certain insulating interaction characteristics suchas protons are implanted between the individual VCSEL elements toproduce high-resistance zones that electrically isolate the VCSELelements. Thus, by controlling ion implantation locations and energies,electrical confinement within a VCSEL and electrical isolation betweenadjacent VCSELs on the same substrate can be implemented.

Prior art ion implantation techniques usually direct ionsperpendicularly or nearly perpendicular onto the surface of a waferbeing implanted. While generally successful, such prior art implantationtechniques are less than optimal in some applications. For example,perpendicular implantation is not suitable for laterally implantinglarge vertical aspect ratio mesa structures. Another limitation is thedifficulty of implementing desired gain guide isolation steps. Finally,prior art ion implantation techniques induce substantial lattice damagein a device's aperture or electrical contact region. Such lattice damagecan be highly detrimental to electrical or optical performance.

Because of the foregoing problems, a new ion implantation techniquewould be beneficial. Particularly beneficial would be an ionimplantation technique that is suitable for use with devices havinglarge vertical aspect ratio mesa structures. Also beneficial would be anion implantation technique that avoids or reduces the problems relatedto obtaining required gain guide isolation steps. Also beneficial wouldbe an ion implantation technique that reduces lattice damage in theaperture or electrical contact region near the aperture.

SUMMARY OF THE INVENTION

Accordingly, the principles of the present invention are directed to anew ion implantation technique. In particular, the principles of thepresent invention provide for angled ion implantation. Such angled ionimplantation can be implemented using a tilted, rotating ionimplantation plate that holds a device (substrate or sample) beingimplanted. The ion implantation plate can be tilted at an angle orangles relative to an ion flux while the device is rotated. This canachieve novel and useful ion implantations in various semiconductordevices, specifically including VCSELs.

According to the principles of the present invention, if a semiconductordevice being ion implanted includes a mesa structure, ion implantationcan be performed below the mesa structure and/or through the mesastructure. In either event, ion damage to the lattice along the apertureregion of the device can be reduced.

Ion implanting a flat semiconductor wafer according to the principles ofthe present invention includes coating the semiconductor wafer with amask material that shields selected areas of the semiconductor waferfrom implanting ions. The mask material is then lithographically exposedand developed. The lithographically masked semiconductor wafer is thenplaced inside an ion implant chamber on a tilted rotation plate. Thetilted rotation plate is then rotated during ion implantation to producean angled ion implant into the semiconductor wafer. Multiple ionimplantations can be performed as desired. Ion implanting asemiconductor wafer having a mesa structure according to an aspect ofthe present invention includes coating the semiconductor wafer with aplanarizable material, such as BCB, polyimide, photo-resist, or spin-onglass. Such planarization is beneficially performed such that theplanarizable material becomes level with the mesa. The planarizablematerial and exposed semiconductor surface is then coated with a maskmaterial that blocks implanting ions. The mask material is thenpatterned, beneficially by lithographic exposure and development. Then,the planarizable material is removed to expose the sidewalls of the mesastructure and part of the semiconductor wafer. Then, the semiconductorwafer is placed in an ion implant chamber on a tilted rotation plate.The rotation plate is then rotated during ion implantation to produce anangled ion implant into the semiconductor wafer. Multiple ionimplantations can be performed as desired, as ion implantation typicallyinvolves multiple implant energies (1 to 5 energy levels are typical)and predetermined dose amounts at each energy level. For example, an ionimplantation could involve the following sequence:

-   -   1^(st) dose: 5E15 ions/cm² at 35 keV    -   2^(nd) dose: 5E15 ions/cm² at 70 keV, etc..        Finally, the mask material and any residual planarization        material is removed.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thatdescription, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 illustrates a typical vertical cavity surface-emitting laser;

FIG. 2 illustrates ion implantation of a vertical cavity surfaceemitting laser wafer according to the principles of the presentinvention;

FIG. 3 illustrates an ion implantation profile of a VCSEL wafer afterimplantation using an implant mask;

FIG. 4 illustrates an ion implantation profile of a mesa-structuredVCSEL wafer after implantation using an implant mask;

FIG. 5 illustrates an ion implantation profile of a VCSEL wafer afterimplantation using an implant mask and multiple energy ion implantationbeams that are directed at different angles; and

FIG. 6 illustrates an ion implantation profile of a VCSEL wafer afterimplantation using multiple implant masks and multiple energy ionimplantation beams.

Note that in the drawings that like numbers designate like elements.Additionally, for explanatory convenience the descriptions usedirectional signals such as up and down, top and bottom, and lower andupper. Such signals, which are derived from the relative positions ofthe elements illustrated in the drawings, are meant to aid theunderstanding of the present invention, not to limit it.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The principles of the present invention provide for ion implantation ofa semiconductor wafer by mounting the semiconductor wafer on a rotatingplate that is tilted at an angle relative to an oncoming ionimplantation flux. The rotation angle or angles and the implantationenergy can be controlled to produce desired implantation profiles anddepths. Furthermore, one or more implantation masks can be used toprovide specific tailoring schemes. Additionally, if the semiconductorwafer being implanted has a mesa structure, ion implantation can occureither through the semiconductor wafer below the mesa or through themesa's wall.

An apparatus 200 for ion implanting a semiconductor wafer, specificallya VCSEL wafer 209, is illustrated in FIG. 2. As shown, the VCSEL wafer209 is mounted on a rotating plate 211. The rotating plate 211 is set ata desired angle relative to an ion implantation flux 207 from an ionsource 205. The angle of the rotating plate 211 is fixed by a tiltmechanism 215. A motor 213 provides rotational force for the rotatingplate 211. As shown in FIG. 2, the ion source 205, the tilt mechanism215, the rotating plate 211, the motor 213, and the VCSEL wafer 209 areall located in an ion chamber 203 during ion implantation.

In operation, the VCSEL wafer 209, which may similar to those that aresubsequently described, is mounted on the rotating plate 211. That plateis set at the desired angle by the mechanism 215. The motor 213 thenrotates the rotating plate 211, beneficially at a constant,pre-determined rotational velocity. The ion source 205 then emits theion flux 207 onto the VCSEL wafer 209 such that ion implantation occurs.It should be understood that the ion implantation energy, dose, and thetilt angle can be controlled to produce a desired implantation profile.It should be further understood that control of the incident ion fluxangle is preferentially controlled by adjusting the angle of the platethat holds the sample. However, it is also possible to control theincident flux angle by either controlling the ion source or controllingthe ion beam. The ion beam can be directed by use of magnets.

While the foregoing has described ion implanting a VCSEL wafer 209, inpractice, other types of semiconductor wafers can also benefit fromangled ion implantation. However, because VCSELs can have ionimplantation-induced apertures, and because such apertures can benefitfrom the principles of the present invention, various ion implantationschemes into VCSEL structures will be provided. However, it should beunderstood that other ion implantation schemes in VCSELs are possible.

FIG. 3 illustrates a planar VCSEL structure 300 having an implantationmask 311. The implantation mask 311 is beneficially formedlithographically. First, a masking of an ion-flux resistant material isformed on the surface of the VCSEL wafer. Then, the mask is patternedlithographically, exposed, and developed.

Referring now to both FIGS. 2 and 3, with the implantation mask 311 inplace, the VCSEL wafer 209 (in FIG. 2, 300 in FIG. 3) is placed in theion chamber 203 on the rotating plate 211. The tilt mechanism 215 isadjusted to set the desired angle between the rotating plate 21 land theimplantation flux 207. The motor 213 is turned on and then ions 315 (seeFIG. 3) are implanted into VCSEL wafer 300.

Turning back to FIG. 3, the VCSEL wafer 300 is comprised of an upperdevice layer 307 (which may include a top DBR 24 as shown in FIG. 1), alower device layer 305 (which may include an active layer 20 as shown inFIG. 1), and a substrate layer 303 (which may include a substrate 12 asshown in FIG. 1). The oncoming ions implant into the VCSEL 300 to forman implant region 307B. As shown, the implantation mask 311 causes theimplant region 307B to have a pointed, wedge-shape cross-section. Thisis beneficial because the lattice damage in the aperture 307A (the areabetween the wedge points in FIG. 3) of the upper device layer 307consequently suffers only minimal ion implant damage. This is highlyadvantageous as the current guide nature of the aperture is enhanced,which improves performance, while the electrical contact and opticalaperture are protected. Thus, the aperture 307A of the VCSEL can betailored by controlling the size of the implantation mask 311, the anglebetween the surface of the VCSEL wafer 300 and the incoming ions 315,and the implantation energy of the incoming ions 315.

The principles of the present invention are also applicable tomesa-structured wafers. For example, FIG. 4 illustrates amesa-structured VCSEL wafer 400 having an implantation mask 411. Theimplantation mask 411 is beneficially formed lithographically. First, aplanarizing material 412 (shown in dotted lines because FIG. 4illustrates the VCSEL wafer 400 after the planarizing material 412 hasbeen removed) such as BCB, polyimide, photo resist, or spin-on glass, iscoated over the VCSEL wafer 400 up to the top of the mesa-structure 407.Then, the VCSEL wafer 400 is coated with a mask material comprised of anion-flux resistant material. Then, the mask material is lithographicallypatterned, exposed, and developed to produce the implantation mask 411.Then, the planarizing material 412 is removed. As shown, it can bebeneficial for the implantation mask 411 to extend over the mesastructure 407.

Referring now to both FIGS. 2 and 4, with the implantation mask 411 inplace, the VCSEL wafer 400 is placed in the chamber 203 on the tiltmechanism 215. The tilt mechanism 215 is then adjusted such that thedesired angle between the rotating plate 211 and the ion flux 207 isachieved. The motor 213 is then turned on while ions 415 (see FIG. 4)are implanted at the desired angle toward the VCSEL wafer 400.

Turning back to FIG. 4, the VCSEL wafer 400 is comprised of a mesa upperdevice layer 407 (which may include a top DBR 24 as shown in FIG. 1), alower device layer 405 (which may include an active layer 20 as shown inFIG. 1), and a substrate layer 403 (which may include a substrate 12 asshown in FIG. 1). The incoming ions implant into the VCSEL 400 to forman isolation region 405B that extends from the lower layer 405 into partof the upper layer 407. The non-implanted region 405A of the lowerdevice layer 405, and the shaded area of the mesa upper device layer 407include an aperture. That aperture can be tailored by controlling thedimensions of the implantation mask, the size, height, and materialcomposition of the mesa-structure 407, the angle between the surface ofthe VCSEL structure 400 and the dose of the incoming ions 415, and theenergy of the incoming ions 415.

The foregoing has described ion implantations of both planar andmesa-structured semiconductor wafers (specifically VCSELs) using only asingle ion irradiation. However, the principles of the present inventioninclude multiple ion irradiations. For example, FIG. 5 illustratesmultiple ion irradiations of a planar VCSEL structure 500. As shown, theVCSEL structure 500 includes an implantation mask 511 that isbeneficially formed lithographically. The VCSEL structure 500 furtherincludes at least an upper layer 505 (which may include a top DBR 24 asshown in FIG. 1) and a lower layer 503 (which may include an activelayer 20 and a substrate 12 as shown in FIG. 1).

Referring now to both FIGS. 2 and 5, with the implantation mask 511 inplace, the VCSEL structure 500 is placed into the ion chamber 203 on therotating plate 211. The tilt mechanism 215 is then adjusted to fix therotating plate 211 at a desired first angle relative to the oncoming ionflux 207 (which is used for the ions 517 and 519 in FIG. 5), which iscontrolled to have a desired first ion implantation energy. The motor213 is then turned on and the ions 517 are directed toward the VCSELstructure 500 (see FIG. 5). The result is an implant region 514 having apointed, wedge-shaped implant cross-section.

Still referring to both FIGS. 2 and 5, after the implant region 514 isformed, the tilt mechanism 215 is adjusted to set the rotating plate 111at a second angle relative to the ion flux 207, which is now controlledto have a desired second ion implantation energy (which is greater thanthe first ion implantation energy). The motor 213 is turned on and ions519 are directed toward the VCSEL structure 500 (see FIG. 5). The resultis an implant region 509 having a pointed, wedge-shaped implantcross-section. As shown the VCSEL structure 500 then has two implantregions, both of which are wedge shaped.

While FIG. 5 shows one method of obtaining multiple wedge-shaped implantregions, the principles of the present invention are broad enough toprovide for another method for obtaining such implant regions. Forexample, FIG. 6 illustrates the result of multiple ion irradiations of aplanar VCSEL structure 650. As shown, the VCSEL structure 650 uses twoimplantation masks, a top implantation mask 651 and a lower implantationmask 657. The implantation masks are located on a VCSEL wafer that iscomprised of at least an upper layer 655 (which may include a top DBR 24as shown in FIG. 1) and of a lower layer 653 (which may include anactive layer 20 and a substrate 12 as shown in FIG. 1). The implantationmasks 651 and 657 are beneficially formed lithographically.

Referring now to both FIGS. 2 and 6, with the implantation masks 651 and657 in place, the VCSEL structure 650 is placed in the ion chamber 203on the rotating plate 211. The tilt mechanism 215 is then adjusted tofix the rotating plate 211 at a pre-determined angle relative to the ionflux 207, which is controlled to have a predetermined first ionimplantation energy. The motor 213 is turned on and ions 619 aredirected toward the VCSEL structure 650 (see FIG. 6). The result is afirst implant region 659 having a pointed, wedge-shaped implantcross-section.

Still referring to both FIGS. 2 and 6, after the implant region 659 isformed, the VCSEL structure 650 is removed from the rotating plate 211and the top implantation mask 651 is removed. The VCSEL structure 650 isthen returned to the rotating plate 211 and then ions 617 are directedtoward the VCSEL structure 650. The result is a second implant region660 having a pointed, wedge-shaped implant cross-section.

As previously noted, the principles of the present invention are highlyuseful. For example, those principles enable implantation of largevertical aspect ratio mesa structures. Additionally, it is possible toprovide an angled current confinement region that is defined byisolation steps. Additionally, lattice damage in a device's aperture andelectrical contact area caused by ion implantation can be reduced.

The embodiments and examples set forth herein are presented to explainthe present invention and its practical application and to therebyenable those skilled in the art to make and utilize the invention. Thoseskilled in the art, however, will recognize that the foregoingdescription and examples have been presented for the purpose ofillustration and example only. Other variations and modifications of thepresent invention will be apparent to those of skill in the art, and itis the intent of the appended claims that such variations andmodifications be covered. The description as set forth is not intendedto be exhaustive or to limit the scope of the invention. Manymodifications and variations are possible in light of the above teachingwithout departing from the spirit and scope of the following claims. Itis contemplated that the use of the present invention can involvecomponents having different characteristics. It is intended that thescope of the present invention be defined by the claims appended hereto,giving full cognizance to equivalents in all respects.

1. An ion-implanted semiconductor device comprising: a substrate layer;a first layer formed over the substrate layer, the first layer includingan active layer; a mesa structure formed over the first layer, at leasta portion of the mesa structure having a substantially rectangularcrosssectional profile; and an ion implantation region formed in both aportion of the mesa structure and a portion of the first layer, whereinthe ion implantation region extends into the portion of the mesa havinga substantially rectangular cross-sectional profile.
 2. An ion-implantedsemiconductor device according to claim 1, wherein the ion-dopedsemiconductor device is a vertical cavity surface emitting laser and themesa structure includes a distributed Bragg reflector.
 3. Anion-implanted semiconductor device according to claim 1, wherein themesa structure further comprises an aperture defined in part by the ionimplantation region and wherein the ion implantation region fanned inthe portion of the mesa structure has a wedge-shaped ion-dopingcross-sectional profile.
 4. Art ion-implanted semiconductor devicecomprising: a substrate layer; a lower layer formed over the substratelayer, the lower layer including an active layer; an upper layer formedover the lower layer; and a first ion implantation region and a secondinn implantation region, at least a portion of each ion implantationregion being defined in the upper layer and/or the lower layer, anon-overlapping portion of the first ion implantation region formedvertically to a non-overlapping portion of the second ion implantationregion and the first and second ion implantation regions defining acurrent confinement region, each of the first and second ionimplantation regions having a substantially wedge-shaped ion-dopingcross-sectional profile, the first ion implantation region associatedwith a first ion implantation energy and the second ion implantationregion associated with a second ion implantation energy.
 5. Anion-implanted semiconductor device according to claim 4, wherein theion-doped semiconductor device is a vertical cavity surface emittinglaser and the upper layer includes a distributed Bragg reflector.
 6. Anion-doped semiconductor device according to claim 5, wherein the upperlayer includes a mesa structure, at least a portion of the mesastructure included in one of the plurality of ion implantation regions.7. An ion-implanted semiconductor device according to claim 4, in whichat least a portion of each ion implantation region is defined in theupper layer.
 8. An ion-implanted semiconductor device according to claim4, in which at least one of the wedge shaped profiles farms a point withan acute angle.